This invention relates in general to handling extended lengths of thin sheet material, such as paper, and more particularly to a device for detecting a splice or other variance in thickness of such material.
Most magazines and many other publications as well are printed on high speed offset presses. The typical press applies a succession of identical printed impressions to a continuous web of paper which passes through the press at a high velocity - often exceeding 1000 ft/min. In addition to the print stands where the ink is actually applied to the web, the typical press includes a dryer which dries the ink, and a former board over which the web is folded and perhaps combined with another web such that the printed impressions on the two webs are in registration. A cutter exists at the end of the press, and here the combined webs are severed between their successive printed impressions to provide individual signatures which are stacked one upon the other. These signatures are later combined with other signatures derived from other runs on the same press or from runs on a different press, and the combined signatures are bound together to produce the magazine or other publication.
The web pays off of a roll of paper large enough to last for 20 to 30 minutes, notwithstanding the high velocity. To avoid rethreading the web once the roll from which it derives is depleted, i is the common practice to splice the end of the web from a depleted roll to the beginning of the web from the new roll. Thus, no interruption in the operation of the press occurs, other than perhaps a reduction in the web velocity to permit the formation of the splice.
The splice advances through the press and rarely falls between printed impressions. But to allow a signature containing a splice to be embodied in an actual magazine is totally unacceptable. The typical procedure for eliminating signatures containing splices involves simply placing a pencil against the web, normally where the web passes over the former board, when splice advances through the press. The splice falls within the group of marked signatures, which usually amount to about 100, and by discarding the entire group, one is assured of eliminating that one signature containing the splice. Many other signatures are eliminated as well-needlessly.
Heretofore, optical sensors have been installed along the web paths of printing presses to detect splices, but these devices have not proved to be very satisfactory. For example, fluctuations in the distance of the web from the sensor will on occasion trigger such a device, providing a false indication of a splice. Being sensitive to the thickness of the paper, these sensors must be adjusted to accommodate webs of varying thickness, and this represents an additional inconvenience.
The splice detector of the present invention directs sound through a rapidly moving web. While the web attenuates the sound waves, a splice in the web will attenuate the sound waves still further, at least for an instant, and this further attenuation is sensed and correlated to the web which is marked for a short distance ahead of the splice and likewise for a short distance beyond the splice. The signatures which bear the markings are thereafter removed, but these signatures are quite few in number. Of course, the splice is within one of those signatures. The device automatically compensates for varying thickness of paper.